The present invention relates to rotating machines and more particularly to intermittent duty rotating machines having an air gap formed between a rotor and a stator within which ice may form.
The present invention provides an apparatus, system and method for preventing ice formation in an air gap between the rotor and stator of an intermittent duty rotating machine such as an electric generator, hydraulic pump or combination of the two devices for the production of power.
Modern aircraft typically include several types of primary power systems that are often driven by an aircraft engine during normal operation. These types of power include electrical, hydraulic and pneumatic, and are used to energize a variety of devices. Many modern multi-engine aircraft also feature emergency power systems for converting energy from one type to another, with a common one being driven by an air turbine, in the event the aircraft loses all or a significant portion of its electrical, hydraulic, or other type of power. The emergency power system is capable of deploying a ram air turbine (RAT) into the air stream surrounding the aircraft in flight. RATs are well known in the art and are disclosed in U.S. Pat. No. 5,746,580, issued May 5, 1998; U.S. Pat. No. 5,558,495, issued Sep. 24, 1996, the disclosures of which are incorporated herein by this reference. Even though all or a substantial portion of the electrical or hydraulic power has been lost, the RAT can still be deployed into the air stream to drive a rotating machine. The rotating machine produces the emergency power necessary to energize the central aircraft functions. This emergency energy is sufficient to retain some degree of control of the aircraft so long as essential control surfaces are still functional. Such rotating machines driven by a RAT include electric generators, hydraulic or pneumatic pumps or devices which combine all or portions of an electric generator and hydraulic or pneumatic pump. Even if the engines driving the primary power systems of the aircraft cease to operate, an emergency power system employing one or more RATs allows an aircraft having sufficient operable flight control surfaces to continue controlled flight with the purpose of seeking a safe landing.
Other devices exist, on board modern aircraft, which are used to convert energy from one form to the other, for the purpose of backing up primary systems. For instance, Hydraulic Motor Generators (HMGs) are used to generate electricity from hydraulic power, to provide an additional channel of electrical power should the generator driven by the main engine become inoperative. Another common application is to use Electric Motor Pumps (EMPs) to covert electrical energy to either hydraulic or pneumatic power. These devices comprise hydraulic or pneumatic pumps driven by electric motors, and are used in the event the engine-mounted devices become inoperative. These devices are mentioned herein because they are not driven by a RAT, but are still used for emergency power conversion.
Emergency power systems, by definition, are infrequently used. Emergency power systems are typically employed in situations in which the system is automatically or manually deployed. While modern aircraft are highly efficient and reliable, emergency power systems are necessary to provide an added degree of redundancy and thereby safety. Since the power system is infrequently, if ever, used under normal circumstances, it remains idle and non-functioning for extended periods of time. Nevertheless, the emergency power system must be able to perform without notice in the event of an emergency resulting from loss of the primary power systems. In such a situation, the emergency power system must go from a dormant condition to a fully operational condition in a few seconds.
It is while the emergency power system is in the dormant condition, i.e. under normal aircraft operating conditions, that potential problems might develop or occur. During normal operation, aircraft components are subjected to a wide range of temperatures and pressures. The ability of air to retain water in the form of water vapor (the saturation vapor pressure) is highly dependent on temperature and pressure. Air having a partial vapor pressure of water vapor below the saturation vapor pressure at one temperature and pressure, when subjected to lower temperatures and pressures may develop a partial vapor pressure of water vapor that would exceed the saturation vapor pressure of water vapor if the water remained in gaseous form inducing condensation of the water from vapor to a liquid. For example, when the aircraft is subjected to several flight cycles, under normal use, condensed water may develop within unventilated enclosures. In the event that condensed water develops, transition from an airport to flight elevation may cause the liquid to freeze. The development or accretion of ice is common in aircraft components during flight operations especially in small unventilated spaces which prevent the escape of moisture. Rotating machines of emergency power systems are typically designed to include such unventilated spaces between the rotor and stator to prevent solid contaminants from fouling operation.
Rotating machines, such as generators, turbines, motors and pumps, include an air gap defined by the space between the rotor and stator, which may be annular, conical, or axial, depending on machine configuration. Accretion of ice in the air gap between the rotor and a stator impairs the ability of the rotor to rotate relative to the stator and therefore, the ability of the rotating machine to generate power. Since the air gap is generally not easily ventilated, ice may form between these two components within the air gap as a result of condensation and subsequent freezing of water vapor from air trapped in the air gap. The ice may develop to a state where it impairs operation of the rotor and stator thereby preventing the generation of electricity, hydraulic pressure or pneumatic pressure by the rotating machine.
Different devices and methods for solving ice formation problems in turbomachinery have been conceived. Examples of such devices and methods are disclosed in U.S. Pat. No.: 5,746,580, issued May 5, 1998; U.S. Pat. No. 5,623,821, issued Apr. 29, 1997; U.S. Pat. No. 5,558,495, issued Sep. 24, 1996; U.S. Pat. No. 5,281,091, issued Jan. 25, 1994; U.S. Pat. No. 5,167,488, issued Dec. 1, 1992; U.S. Pat. No. 4,747,748, issued May 31, 1988; and U.S. Pat. No. 3,834,157 issued Sep. 10, 1974, the disclosures of which are incorporated herein for their teachings regarding turbomachinery.
Some additional prior deicing schemes have attempted to prevent ice formation by placing heating blankets wherever convenient within the cavity of a generator housing. Such heating blankets are disposed remotely from the rotor and stator. Such heating blankets are not placed within the air gap. The heating blanket is a form of heating element which is used to retrofit an emergency system. Because the heating blankets do not focus on the specific area requiring deicing, i.e. the air gap, they may not be thorough or efficient in removing ice. While this may provide acceptable results in some situations, it is desirable to further reduce the possibility of ice formation and to eliminate ice in the event it has already formed.
An air gap deicing device for use with a rotating machine having a stationary stator separated by an air gap from a rotating rotor rotating about an axis of rotation extending therethrough in an axial direction in accordance with one aspect of the disclosure includes a heating element and a thermally conductive shield disposed between the heating element and the air gap. The heating element is disposed in at least one of the rotor and stator adjacent the air gap. The deicing device may also include a thermal shield disposed between the heating element and a portion of the one of the rotor and stator. Additionally, if the one of the rotor and stator is formed to include a plurality of apertures extending therethrough in the axial direction, the heating element may extend through the plurality of apertures. Heating element may be an electrical resistive heating element.
A rotating machine in accordance with another aspect of the disclosure includes a stator, a rotor configured for rotational movement relative to stator and separated from stator by an air gap, and an electrically resistive heating element disposed in one of the stator and rotor in thermal communication with the air gap. The machine may also include a thermal shield disposed between the heating element and portions of the one of the rotor and stator separated from the air gap by the heating element. Additionally, the rotating machine may include a thermally conductive element in thermal communication with the heating element and disposed between the heating element and the air gap.
A method of inhibiting ice formation in a rotating machine in accordance with an aspect of the disclosure includes the steps of providing a rotating machine having coaxially arranged rotor/stator combination including a rotor and a stator defining an air gap therebetween, providing an electrically powered resistive heater, disposing the provided resistive heater within one of the rotor and stator adjacent the air gap, energizing the provided resistive heater. A temperature sensor may be provided adjacent the air gap to sense the temperature of air in the air gap so that the resistive heater is energized when the sensed temperature is at or below a selected limit and de-energized when the sensed temperature is above the selected limit.
Additional features and advantages of the invention will become apparent to those skilled in the art upon a consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.